I have been getting low recoveries on my GGA for CBOD. I purchased the book "Laboratory Testing for BOD and CBOD" by Brake and Raynovic, to try to troubleshoot my problem. On page 13, it says that the goal for the average of repeated CBOD results for the GGA standard is 171mg/L. If I use this as the basis for my GGA CBOD recoveries, all of my recoveries would be OK. I currently use the 198mg/L for calculating recovery. I use SM5210B 19th Edition, it does not give a value for GGA CBOD recovery.

My question is where did the 171mg/L number come from?

I would like to reference something if I change my SOP to reflect this number.

The 171 mg/L value was calculated by comparing the average BOD and CBOD results on QC studies of G-GA standards and using that ratio (1.16) to factor the 198 mg/L BOD to 171 CBOD. Table 1 of the Brake and Raynovic publication contains the results of that summary. It is explained on page 13 of that book.

My experience with BOD and CBOD is that we get almost the same value with and without inhibitor. We use a dried seed material that has no nitrifiers. With settled influent the BOD values will be up to 220 mg/L while CBOD is still about 190 mg/L. I have had low results for CBOD if the standard is not pH neutralized while that did not affect the BOD result.

It appears that Standard Methods still uses the same 198 mg/L values as the organic content of the G-GA standard is the same for BOD and CBOD.

James Royer is correct, DRajchel...if your are testing a sample that contains nitrogen (for example, glucose and glutamic acid, the latter of which contains nitrogen), AND you are using a seed that has no nitrifying microorganisms (such as a commercial seed), you will get lower results than if you had used a seed WITH nitrifiers. If the sample contains no nitrogen, you should theoretically get the same results for BOD as you do for CBOD, regardless of whether or not there are nitrifiers (including inhibited nitrifiers) in the seed.

In doing the research that resulted in the 171 mg/L goal for a 198 mg/L glucose and glutamic acid (a 50:50 mixture of the two compounds), I reviewed the results for BOD and CBOD in EPA's discontinued Water Pollution (WP) studies (I reviewed only the 21 results for WP023 through WP040). The "demands" samples for those WP studies were all composed of 50:50 mixtures of G and GA (as PE samples still are today). The ratio of BOD/CBOD was a very steady 1.16. I concluded from this that a GGA sample that would normally would have a BOD of 198 would have a CBOD of 198/1.16 or 171 mg/L.

So that explains where the 171 mg/L came from, but if you want to have EPA's blessing support your SOP change, a better reference would be the EPA/NELAC "interpretation" document for the CBOD goal, the text of which I have attached here.

Therein, EPA says a lab should establish its own goal for the CBOD of the GGA standard. They place a lower limit of 150 mg/L, which, if attained during a batch, would disqualify all data for that batch. Once you have established your lab's average GGA result, the acceptance limits are calculated as ± 2 and 3 standard deviations from the average/mean where the standard deviation cannot exceed 7.5% of the average.

You may find that those regulating the operation of your lab may not be aware of, or may choose to disregard the EPA interpretation document. My advice to you would be to check with them before doing a lot of work on your SOP only to have it thrown out on the first inspection. Some states, for example, say "you must follow Standard Methods" and it is difficult to get them out of that mind set even with the support of an EPA document saying otherwise.